The concept of a "combined gas/steam turbine power station plant"--hereafter referred to as a combined plant--is always understood, in the following, to mean a gas turbine group with at least one downstream steam turbine cycle, the gas turbine exhaust heat being utilised in an exhaust heat boiler to produce the steam necessary to supply a steam turbine.
As is known from German Pat. No. 2,524,723, the exhaust heat steam generator is equipped with auxiliary firing equipment to increase the steam turbine power. This auxiliary firing equipment and the gas turbine combustion chamber are fired by a fuel gas which is obtained from a gas generator operating by coal gasification.
This cycle has the disadvantage, however, that air is used for the coal gasification, with the effect that a fuel gas of low calorie value is obtained. A further disadvantage of this cycle lies in the fact that the high gas turbine entry temperature, together with a high pressure ratio to improve the process efficiency, lead to a high NO.sub.X emission rate. The same also applies to the auxiliary firing in the exhaust gas steam generator.
Now gas turbines are increasingly subject--with respect to exhaust gas composition--to the strict environmental protection regulations of many states. In particular, great difficulties are involved in keeping within the maximum permitted NO.sub.X emission regulations. Thus, legal regulations are currently in force--in the USA, in particular--which state that the NO.sub.X emission content must not exceed approximately 75 ppm at 15% O.sub.2 by volume. Similar regulations have to be observed in most industrial states and it is to be expected that the permissible emission figures will tend to be corrected downwards in future.
It is known from the journal "Brown-Boveri-Mitteilung", Volume 65, CH-Baden, October 1978, in particular page 649, that water or a quantity of steam extracted from the process may be respectively injected or introduced into the gas turbine combustion chamber as an NO.sub.X reduction measure.
If water is injected into the combustion chamber, a reduction in the flame temperature and, in consequence, a noticeable deterioration of the efficiency of the overall process is to be expected. Furthermore, water is not always and everywhere present in usable quantities-- in low precipitation regions, for example--particularly if use of the water available locally to maintain the combined process has to be given priority. In addition, the water requires treatment before it can be used because many of the minerals present in the water, such as sodium, common salt, etc., have a very corrosive effect on the gas turbine. This treatment is, however, expensive.
The introduction of steam extracted from the process also leads to a deterioration in overall efficiency, even if the gas turbine entry temperature can be kept up using steam addition. Although the addition of steam to the gas turbine process produces an increase in total power, because of the additional mass flow, the result is, however, a reduction in efficiency because the additional consumption of fuel must be taken into account.
German Pat. No. 2,524,848 reveals the supply of a part of the compressed combustion air to the air decomposition installation, in which the air is separated into oxygen and nitrogen. The oxygen is provided for the operation of the gas producer and, the nitrogen is conducted into the gas turbine combustion chamber. Although this cycle permits a reduction in the NO.sub.X generated, it is, however, only a partial reduction. Furthermore, since these gas turbine exhaust gases are fed to the auxiliary firing equipment in the exhaust heat steam generator, the combustion occuring there again increases the NO.sub.X emissions to inadmissible values.
The invention is intended to help deal with the foregoing problem.
The objective of the invention is to minimise the NO.sub.X emissions in a combined plant of the type stated above, by integral nitrogen mixing.
Since, on the one hand, the gas turbine exhaust gases have a reduced NO.sub.X figure and, on the other, a mixture of fuel gas and nitrogen is employed for auxiliary firing in the exhaust heat steam generator, optimum minimisation of the NO.sub.X emissions can be achieved.
In further preferred embodiments, the fuel gas and nitrogen mixture is passed through one expansion turbine before firing in the combustion chamber and through another expansion turbine before auxiliary firing in the exhaust heat steam generator. The expansion turbines can drive generators or compressors. Where air compressors are used, the air decomposition plant can, in part, be mechanically driven.